The Peripheral Component Interconnect (PCI) standard describes a computer bus for connecting peripheral devices to a computer motherboard. These peripheral devices typically take the form of an expansion card or other such device. PCI standard covers the physical size of the bus, electrical characteristics, bus timing, and protocols required for communicating over the bus.
The basic PCI standard has some shortcomings that prevent it from providing the bandwidth and features needed by current and future generations of I/O and storage devices. One such problem is its highly parallel, shared-bus architecture that limits its bus speed and scalability. Also, its simple, load-store, flat memory-based communications model is less robust and extensible than a routed, packet-based model.
PCI Express (PCIe), a computer expansion card interface format, was designed to address some of the problems of the basic PCI standard. Unlike the PCI interface, rather than being a bus, the PCIe interface is structured around point-to-point pairs of serial (1 bit), unidirectional links, also referred to as lanes. This is in contrast to the PCI standard that is a bus-based system in which all the devices share the same bidirectional, 32-bit (or 64-bit), parallel signal path.
In PCIe's point-to-point bus topology, a shared switch replaces PCI's shared bus as the single shared resource by which all of the devices communicate. Unlike in the shared bus topology, where the devices must collectively arbitrate among themselves for use of the bus, each device in the PCIe system has direct and exclusive access to the switch. In other words, each PCIe device is connected to its own dedicated lane.
PCIe implements a serial, point-to-point type interconnect for communication between two devices. Multiple PCIe devices are interconnected via the use of switches which means one can practically connect a large number of devices together in a system. A point-to-point interconnect implies limited electrical load on the link allowing transmission and reception frequencies to scale to much higher numbers. Currently PCIe transmission and reception data rate is 2.5 Gbits/sec. A serial interconnect between two devices results in fewer pins per device package which reduces PCIe chip and board design cost and reduces board design complexity. PCIe performance is also highly scalable. This is achieved by implementing scalable numbers for pins and signal Lanes per interconnect based on communication performance requirements for that interconnect.
PCIe implements switch-based technology to interconnect a large number of devices. Communication over the serial interconnect is accomplished using a packet-based communication protocol. Quality of Service (QoS) features provides differentiated transmission performance for different applications. Hot Plug/Hot Swap support enables “always-on” systems. Advanced power management features allow one to design for low power mobile applications. RAS (Reliable, Available, and Serviceable) error handling features make PCI Express suitable for robust high-end server applications. Hot plug, power management, error handling and interrupt signaling are accomplished in-band using packet based messaging rather than side-band signals. This keeps the device pin count low and reduces system cost.
Mass storage devices (i.e., solid state flash memory, optical drives, magnetic disk drives) typically use the Serial Advanced Technology Attachment (SATA) standard for communicating with the host computer. In fact, the SATA standard was primarily designed for the transfer of data between the host computer and the mass storage device at data rates of 1.5-3.0 Gbps. SATA's main advantages over the older, parallel ATA interface are faster data transfer, ability to remove or add devices while operating (hot swapping), and more reliable operation with tighter data integrity checks.
In order for a SATA mass storage device to be connected to a host computer's PCIe connection, a PCIe-to-SATA bridge controller is used. The bridge controller emulates a SATA host bus adaptor (HBA). FIG. 1 illustrates a block diagram of a typical prior art computer host system 101 connected to a SATA mass storage device 102 using a PCIe-to-SATA bridge 100. The bridge 100 incorporates an Advanced Host Controller Interface (AHCI) that is a hardware mechanism that allows software to communicate with the SATA device 102. AHCI is a PCIe class device that acts as a data movement engine between the host computer's system memory and the SATA device 102.
FIG. 2 illustrates a block diagram of a typical prior art PCIe-to-SATA bridge controller 100. The bridge controller 100 is comprised of a PCIe PHY block 201 that provides the conversion from the analog nature of the PCIe link to the digital environment of the bridge controller 100. The PHY block 201 also converts the approximately 2.5 Gbps data rate down into the Mbps range.
The PCIe core 202 provides packet processing and decoding. The PCIe-to-SATA core bridge 203 takes the data from the PCIe core 202 and puts it into the SATA standard format. The Application block 204 is responsible for processing the SATA information from the core bridge 203. The SATA Transport block 205 is responsible for management of the frame information structure (FIS) that is the mechanism to transfer information between the host and the device application layer. The SATA Link layer 206 provides SATA standard encoding. The SATA PHY Layer 207 performs the conversion from the digital environment of the bridge controller 100 to the analog environment of the SATA interface. The SATA PHY block 208 is comprised of transmit circuits that provide the 1.5-3.0 Gbps data rate over the SATA bus.
FIG. 3 illustrates a block diagram of a typical prior art SATA mass storage device 102. The storage media 306 is interfaced to the SATA bus using the same layers 301-305 as the above-described bridge controller 100, but in a reverse order. Not only do these redundant functions require additional real estate on the SATA mass storage device 102 to implement, the time required to communicate from the host to the mass storage device is increased due to the redundant conversions necessary to go from PCIe to SATA to the storage media 306.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a mass storage device that communicates over PCIe to reduce latency and cost associated with using an HBA and a SATA storage device.